Enter Route 11 by its western entrance, from Opelucid City . Head east until you reach the bridge enabling people to cross the stream. Surf onto the water north of the bridge. Move over to the waterfall and use Waterfall to head up it.
Once you're at the top, move onto the land to the west first. Move west until you find an item obscured between two trees. Move over to this item and pick it up to find that it's some Zinc ! Loverly! Now move back to the water and **** Surf to the land east of the top of the waterfall.
Use your Dowsing Machine to find the HP Up hidden here. Now move back onto the water once more, head back down the waterfall and move back onto the bridge. This time, head onto the water south of the bridge and **** Surf along the stream, taking you east.
When you reach the waterfall at the end, drop down it to the water below. Move onto the land to your east. There's an Amoonguss here posing as an item. Interact with it if you wish to battle it.
Wild Pokemon Battle
After you're done, head north-west, then head north under the bridge. Once you're on the other side, move up to the Pokemon Ranger here and take her on!
Trainer Battle: Pokemon Ranger
Note: After defeating her, she'll give you an Aspear Berry.
The supposed item to the left is simply another Amoonguss . The supposed item to the right is an actual item, believe it or not. Pick it up to find that it's a Razor Fang . How cool!